Environmental test chambers are widely used to test electrical products such as printed circuit boards and other electrical/electronic products which include a printed circuit board as a component part. Such testing involves monitoring certain electrical operating characteristics of the product while it is undergoing extreme changes in temperature. Temperature cycling over a range of +125.degree. C. to -65.degree. C. (about +255.degree. F. to -85.degree. F.) is not uncommon and is often accompanied by extreme changes in humidity and/or by vibrating the product under test. In the industry, testing of this type is often referred to as "stress testing."
A primary reason that product stress testing is undertaken is to identify particular products (within a larger group of products) which exhibit characteristics evidencing probable premature failure. And such testing is intended to cull out those products which actually fail during test. In the vernacular of the industry, such products are said to exhibit "infant mortality." Those products which do not exhibit infant mortality are much more reliable in the automotive, aircraft, military or other application in which they are used. U.S. Pat. Nos. 3,656,058 (Leathers); 4,683,424 (Cutright et al.); 4,949,031 (Szasz et al.) and 5,021,732 (Fuoco et al.) all disclose apparatus used for environmental stress testing.
Designers of environmental test chambers must deal with a number of engineering considerations. One is the rapidity with which the temperature in the product-containing chamber can be changed. In a test chamber having refrigeration and heating systems of a particular size, the rate at which the temperature can be changed is, in significant part, a function of the mass (e.g., the mass of the products and product carriers) in the chamber. This is so because the heat absorbed by the chamber contents and the heat which must be removed therefrom is a function of such mass.
A household refrigerator is a good analogy of the foregoing. For a particular refrigerator, two pounds of foodstuffs in the refrigerator are more quickly cooled to a particular temperature than twenty pounds of foodstuffs. And as a corollary, cooling twenty pounds of foodstuffs to a particular temperature within a stated time requires a larger refrigeration unit than is required to cool two pounds of foodstuffs to the same temperature within the same time.
Another engineering consideration involves the equipment used to monitor the electrical operating characteristics of the products under test. Such equipment is temperature sensitive and must be maintained nominally at room ambient conditions. In other words, such equipment should not be in the chamber with the products undergoing test.
Still another engineering consideration is whether to configure the test chamber for batch-process or continuous-process testing. The apparatus of above-noted Szasz et al. patent is for batch testing in that a number of products are placed on a pallet which is inserted into the chamber. All the products on the pallet undergo test simultaneously and after such test is completed, the pallet and its "batch" of products is removed and another pallet loaded with products to be tested is inserted.
On the other hand, the vibration chamber disclosed in U.S. Pat. No. 5,226,326 (Polen et al.) may be referred to as a type of continuous testing arrangement. Such chamber uses a conveyor having spaced pairs of rollers to grasp respective edges of flatwise-oriented printed circuit boards to be tested. Similarly, U.S. Pat. No. 5,397,998 (Soeno et al.) discloses several different arrangements of a conveyor and products to be tested carried atop such conveyor. In one arrangement, feeder apparatus along the conveyor supply electric power to the products during burn-in and the "burned-in" products are then tested after exiting at the end of the conveyor.
While these earlier arrangements are thought to have been generally satisfactory for their intended uses, they are not without disadvantages for some types of applications. For example, the conveying arrangements shown in the Soeno et al. patent apparently do not permit instrumented product testing while the product is moving through the chamber. In other words, such instrumented testing is carried out after the product leaves the burn-in chamber. The "failure mode" characteristics exhibited by the products while in the burn-in chamber and after they leave such chamber may differ markedly.
Yet another disadvantage of the conveying arrangements of the Soeno et al. patent is that the mass of the conveyor (as well as that of the product to be tested) is in the burn-in chamber. Chamber temperature changes can be accomplished and maintained only by adding heat to or removing heat from the conveyor components.
Still another disadvantage of prior art arrangements is that they seemingly have not appreciated how to configure test chambers so that the size and capacity thereof can be selected or changed to suit a particular application. For example, the arrangement shown in FIG. 5 of the Polen et al. patent apparently has a fixed length which cannot be changed. At least, there is no suggestion to the contrary.
A new environmental test apparatus which addresses certain shortcomings of earlier apparatus would be an important advance in the art.